The invention relates to a method for transmitting data in a data transmission system which includes a receiver which is from time to time in a stand-by mode and from time to time in a data-receive mode. The receiver can be switched by a switch-on signal from the stand-by mode into the data-receive mode. The data transmission system also includes at least one transmitter which transmits data to the receiver at time intervals. The receiver is in the data-receive mode during data transmission. The invention also is directed to a data transmission system for carrying out the method of the invention.
Methods of the kind described above for transmitting data in a data transmission system are known and are applied, for example, in air pressure control systems. Such air pressure control systems are built into a motor vehicle and comprise essentially a central unit and air pressure control devices. Each of the pressure control devices is assigned to a corresponding one of the wheels of the motor vehicle. With the aid of the air pressure control devices, the air pressure in the corresponding tires of the motor vehicle is detected and the air pressure control devices transmit a data message to the central unit at time intervals. Each data transmission includes an individual identifier in addition to the measured air pressure. With the aid of the individual identifier, a check is made in the central unit as to from which wheel position the data transmission was transmitted. In the central unit, it is, for example, stored that the individual identifier III is transmitted from the wheel position xe2x80x9cforward leftxe2x80x9d.
The central unit compares the transmitted air pressure values to stored desired air pressure values and a warning is transmitted to the driver of the vehicle when the measured air pressure deviates from the measured desired air pressure by an amount greater than a pregiven quantity. If no announcement is sent to the driver, then this is an indication that the correct air pressure is present in all tires of the motor vehicle.
The measurement of the tire air pressure and the announcement of an incorrect tire air pressure is especially important during travel of the motor vehicle. However, it is likewise important that an announcement is made to the driver when starting the vehicle that the correct air pressure is present in all tires and that the driver may commence driving without danger. The air pressure control devices also measure the air pressure in the tires at time intervals when the vehicle is at standstill and transmit this information to the central unit where the air pressure is checked as to correctness. When the motor vehicle is started, the central unit can reliably indicate to the driver, on the basis of the last-transmitted values, whether the air pressure in all tires of the motor vehicle assumes the correct value. From this, it follows that the central unit must be able to receive the data transmissions of the air pressure control devices even at standstill of the motor vehicle. For this purpose, the central unit must be supplied with energy. At standstill of the motor vehicle, this energy is made available by the vehicle battery which, as a consequence of a longer standstill, is possibly loaded until completely discharged.
Data transmission systems of the above-mentioned type are known wherein the energy consumption of the receiver can be minimized. These data transmission systems contain, for example, an electronic ignition key (transmitter) which sends out a signal having an individual identifier to the receiver in the door of the motor vehicle. The receiver initiates opening of the door if the individual identifier, which is transmitted from the key, belongs to the corresponding motor vehicle. In a data transmission system of this kind, the receiver is in general in a stand-by mode wherein it can receive a base signal from the transmitters, that is, from the ignition keys. When a base signal is received, an amplifier circuit is switched on in the receiver and this amplifier circuit checks whether the received base signal exhibits a vehicle-typical character. If this is the case, then a downstream data evaluation unit in the receiver is switched on which compares the individual identifier transmitted from the ignition key to the individual identifier stored in the receiver and, when there is coincidence, an opening of the door is initiated. The battery supplies the central unit with energy and is sparingly used because the energy-consuming amplifier circuit as well as the energy-consuming data evaluation unit are switched on only as needed.
The method explained in the last paragraph for transmitting data in a data transmission system is not transferrable to air pressure control systems because, as initially mentioned, data is continuously transmitted from the transmitters of an air pressure control system even at standstill of the motor vehicle. If a vehicle equipped with a corresponding air pressure control system is parked, for example, in a large parking lot, then at least the amplifier circuit in the central unit is switched on continuously when applying the above-mentioned method because the central unit would very often receive the base signal from the motor vehicles standing in the immediate area. The energy consumption of the central unit therefore cannot be significantly reduced. Furthermore, it can be determined that the base signal must be transmitted from the transmitters over a certain time span so that the amplifier circuit in the central unit can be switched on. In this way, the battery of the transmitters is, in turn, greatly loaded.
It is an object of the invention to provide a method for transmitting data in a data transmission system wherein the energy resources of the receiver as well as the energy resources of the transmitters are used sparingly. It is a further object of the invention to provide a data transmission system with which the method of the invention can be carried out. The data transmission system should especially, but not exclusively, be suitable for use in air pressure control systems.
The method of the invention is for transmitting data in a data transmission system which includes: a receiver which is intermittently in a stand-by mode and intermittently in a data-receive mode; at least one transmitter for making a data transmission to the receiver at time intervals; and, means for supplying a switch-on signal to switch the receiver from the stand-by mode to the data-receive mode; and, the receiver being in the data-receive mode during the data transmission. The method of the invention includes the steps of: inputting a message to the receiver as to the time intervals at which the data transmissions are to be expected from the transmitter; on the basis of the message, determining the time point in the receiver at which the next data transmission can be expected from the transmitter measured from after receipt of the last data transmission; and, generating the switch-on signal in the receiver shortly before the time point at which the next data transmission is to be expected.
In the following, the term xe2x80x9cdata-receive modexe2x80x9d is intended to mean that mode wherein the receiver can receive and completely process the data transmitted by the transmitters. The phrase xe2x80x9cshortly before the time point at which the next data transmission is to be expected, the switch-on signal is generatedxe2x80x9d is understood to mean that the time span between the time point, at which the switch-on signal is generated, and the time point, at which the next data transmission is to be expected, is significantly less (that is, at least by one order of magnitude less) than the time span between two data transmissions of a transmitter. It is further noted that the method for transmitting data in a data transmission system and the data transmission system is explained in connection with an air pressure control system. However, the method as well as the data transmission system can be transferred to all systems wherein data are transmitted from transmitters to a receiver at known time intervals.
The basic idea of the invention is that the receiver of the data transmission system is only switched into the data-receive mode when a data transmission is to be expected from a transmitter and that the switch-on signal, with which the data-receive mode is switched on, is generated in the receiver itself. After reception of the data transmission in the receiver, a switch-off signal is generated in the receiver which transfers the receiver again from the data-receive mode into the stand-by mode.
The advantages achieved by the invention are especially that the receiver of the data transmission system is only then switched on in the data-receive mode when a data transmission is to be expected from a transmitter belonging to the system. The receiver is only then in the energy-consuming data-receive mode when it is absolutely necessary. A further advantage of the invention is that no base signal need be transmitted from the transmitters of the data transmission system with the aid of which the receiver of the data transmission system is transferred into the data-receive mode. Rather, this switch-on signal is generated within the receiver itself and therefore the battery of the transmitters is also sparingly used.
According to a feature of the invention, each transmitter transmits data to the receiver at regular time intervals. The data with respect to the regular time intervals is inputted into the receiver. If the system includes, for example, a transmitter, which transmits a data transmission at regular time intervals of, for example, 60 seconds, to the receiver, then it can be inputted into the receiver during manufacture that data transmissions are to be expected every 60 seconds from the transmitter with which the receiver communicates. If the data transmission system has several transmitters, then each transmitter transmits an identifier (which is individually assigned to the transmitter) with each data transmission. In the receiver, the regular time intervals are stored at which data transmissions are to be expected which contain a specific individual identifier (for example, it is stored in the receiver that a transmitter having the individual identifier A, transmits a data transmission every 60 seconds and the transmitter having the individual identifier B transmits a data transmission every 45 seconds).
According to a feature of the invention, each transmitter sends, at time intervals, data to the receiver and each data transmission contains information at which time intervals the next data transmission is to be expected from a transmitter. In this embodiment too, each transmitter transmits with each data transmission an individual identifier to the receiver when the data transmission system contains several transmitters. The advantage of this embodiment is that it is not necessary to input to the receiver when the next data transmission is to be expected from a specific transmitter because this is given to the receiver from the transmitter. For this reason, it is also very simple to exchange a transmitter in the data transmission system. This is especially important in air pressure control systems because the transmitters there are often exchanged, for example, when changing from summer tires to winter tires.
According to another feature of the invention, the data as to at which time intervals the next data transmission is sent, are generated in the transmitter in accordance with an algorithm. Via the algorithm, a random number is advantageously generated which indicates the time intervals, for example, in seconds. By using an algorithm which generates the random number, it is substantially ensured that the several transmitters of a data transmission system transmit their data transmissions to the receiver always at different time points.
According to another feature of the invention, a comparison is made in the receiver of the time intervals at which the data transmissions are received by the receiver and which time intervals have been indicated by the transmitter. Based on the comparison, a corrective value is computed and stored which permits a time span given by the transmitter to be converted into a time span measured in the receiver. The time span is given by the transmitter. The advantage of this embodiment is that the clocks in the transmitters and in the receiver are calibrated with respect to each other so that the time span from the generation of the input signal in the receiver up to the reception of the data transmission can be minimized. If a transmitter tells the receiver, for example, that the next data transmission is to be expected after 60 seconds and, if in the receiver, a time span of 66 seconds is measured from receipt of the last data transmission to the receipt of the next data transmission from this transmitter, then a corrective value of 1.1 is to be considered by the receiver. If the transmitter provides in the data message, which is received next by the receiver, that the next data message is to be expected after 120 seconds, then the corrective value is considered in the receiver and the receiver knows that with respect to its clock, the next data transmission is to be expected only in 132 seconds. It is sufficient, for example, when the switch-on signal is generated after 131.5 seconds measured with the clock in the receiver in lieu of, for example, after 119.5 seconds which the receiver would use as a basis if it did not consider the corrective value.
According to still another embodiment of the invention, the receiver switches into the data-receive mode and remains there until it receives a new data transmission from a transmitter, if the receiver could not receive a previously expected message from the transmitter. The advantage of this embodiment is that the method can also be further operated when a data transmission from a transmitter is not received. This can, for example, take place when the receiver had switched too late from the stand-by mode into the data-receive mode and, in this way, did not receive the data transmission; or, two or more transmitters transmitted their data transmissions simultaneously to the receiver in a data transmission system having several transmitters. These data transmissions are not separated in the receiver and are therefore not processed.
If the data message of a transmitter contains, for example, the individual identifier A and the information that the next data transmission is transmitted in 45 seconds and if the next data message is not received after 45 seconds by the receiver, then the receiver switches into the data-receive mode until it, in turn, receives a data message which contains the individual identifier A. This data message contains then information as to when the next data message is to be expected so that the receiver can take up its normal mode of operation. If circumstances develop that the receiver cannot receive the data messages from all transmitters at a time interval, then the receiver switches into the data-receive mode until the data messages have been received from all transmitters which have respectively different identifiers.